Swiss Patent 676 177 describes an optoelectronic, finger-operated keyboard
--which is disposed behind a laminated-glass pane of a show window or of a bank counter, said laminated-glass pane being provided with character areas, PA1 --with keys for generating one associated operation signal each, PA1 --with one infrared emitter per key which is disposed at the inside of the laminated-glass pane and emits infrared light which is collimated virtually perpendicular to said inside by means of a diaphragm, and PA1 --with one infrared detector adjacent to each infrared emitter which is pointed at the laminated-glass pane, PA1 --with an operating circuit comprising: PA1 --at least two keys for generating one associated operation signal each, PA1 --an infrared-transmitting cover, PA1 --one infrared emitter per key which is disposed at the inside of the cover and emits infrared light collimated virtually perpendicular to said inside, PA1 --one infrared detector adjacent to each infrared emitter which is separated from the latter by a shield opaque to infrared, and which is pointed at the cover, PA1 --an operating circuit comprising: PA1 --A further improvement in signal-to-noise ratio over the prior art arrangement is obtained through the shield between infrared emitter and detector. PA1 --A reflection at the outside of the cover will not operate the keyboard. Therefore, dew on or contamination of the outside can be permitted. In the prior art arrangement, this would result in false operations. PA1 --The thickness of the cover can be chosen so that the various explosion-protection requirements can be complied with; the glassy cover can therefore be 5 mm thick, but preferably even thicker. PA1 --Because of the vertical position of the infrared emitter and because of the shield, the infrared detector can be much closer to the infrared emitter than in the prior art arrangement, so that the infrared emitters can be arranged side by side with a minimum spacing just sufficient for individual operation. PA1 --Therefore, compact keyboards can be constructed. PA1 --The drop in the intensity of the infrared light emitted by the infrared emitters is reduced to the point that conventional infrared emitters need not be replaced during the service life of the industrial measurement equipment.
----said infrared detector being located within the main radiation range of the infrared light emitted by the associated infrared emitter and diffusely scattered at a finger, but outside the infrared light reflected from the outside and/or inside of the laminated-glass pane, and PA2 ----a separate amplifier and a separate threshold switch for each key, and PA2 ----a generator for current pulses for feeding the infrared emitters. PA2 ----the infrared emitters being arranged side by side with a minimum spacing sufficient for individual operation, PA2 ----said infrared detector being located within the main radiation range of the infrared light emitted by the associated infrared emitter and diffusely scattered at a finger, but outside the infrared light reflected from the outside and/or inside of the cover, and PA2 ----a separate control and detection circuit for each key PA2 ----a scanning circuit common to all keys
With respect to the operating circuit, this prior art only states that the feeding with current pulses is done to achieve a current saving.
The prior art keyboard achieves a rather good signal-to-noise ratio through the spatial arrangement of infrared emitter and detector but does not take into account an essential property of infrared emitters which turns out to be increasingly disadvantageous after prolonged operation, namely the well-known drop in the intensity of the transmitted infrared light with unchanged current strength over protracted operating times. In this connection it is clear that an increase in the transmitting power of the infrared emitter would not improve the long-time signal-to-noise ratio.
As investigations by the inventors have shown, optimizing the signal-to-noise ratio requires, in addition to an optimum spatial arrangement of infrared emitter and detector, which is particularly important with thick covers, i.e., if the thickness of the latter is to exceed 2 mm to 3 mm, and if the infrared emitter and detector are to be close together to achieve a compact design, that the operating circuit should be designed to prevent the above-mentioned drop in intensity.
Furthermore, a thick cover as is necessary to meet the requirements of the various explosion-protection classes for industrial measurement equipment requires a great distance between the finger and the infrared detector, so that the signal level occurring at the detector is reduced as compared with a thin cover. With such thick covers, too, an increase in the transmitting power of the infrared emitter would not improve the signal-to-noise ratio.
The invention as claimed serves to attain these objects in an optimum fashion. Thus, through an optimum mutual arrangement, the spurious-signal level at the infrared detector must be reduced and the detector's useful-signal level must be made as high as possible. At the same time, the drop in the intensity of the transmitted infrared light over the operating time of the infrared emitters must be reduced by a suitable circuit design taking into account the operability, i.e., sufficiently fast and reliable "detection" of a finger operation, so that the initial intensity is maintained as long as possible.